High-Pressure Synthesis And Physical Properties Study Of Ba-based Perovskite Oxide Single Crystals

Alkaline-earth metal perovskites AFe O₃（A=Ca,Sr and Ba）is one of the rare compounds that can stabilize the abnormally high-valent state of Fe⁴⁺ions.These systems exhibit interesting physical properties such as strong p-d hybridization effect,helical magnetic ordering,half-metallic behavior,charge disproportionation and topological spin structure,which are the important frontier in the research of condensed-matter physics.In particular,the helical magnetic structure for cubic BaFe O₃ is greatly affected by the magnetic field and chemical doping,which provides an ideal material system for studying the physical property regulation of high-valent Fe⁴⁺compounds.In this doctoral dissertation,by means of atmospheric pressure floating zone melting single crystal growth method combined with the unique high-temperature and high-pressure technique,large-volume single crystals BaFe O₃ doped with Mn⁴⁺or Co⁴⁺ions at B-site were prepared for the first time.And the crystal structures and comprehensive physical properties of the solid solution systems were studied in detail.The main research results are as follows:（1）Large-volume high-pressure single crystals of BaFe_1-xMn_xO₃（0≤x≤1）solid solution were successfully obtained for the first time by using atmospheric pressure floating zone melting method and high-pressure and high-temperature synthesis technique（two-step method）.The experimental results show that when x<0.3,the solid solution systems have the same simple cubic Pm-3m perovskite structure as the parent phase BaFe O₃.When the content ofMnexceeds 0.3,the solid solution crystallized into BaMnO₃-type 4H hexagonal structure with the P6₃/mmc space group.The intermediate doping component（x=0.3）shows the phase separation which means that the cubic structure coexists with the hexagonal structure.By measuring ac/dc magnetic susceptibility and specific heat,the phase diagram of multiple magnetic transitions of solid solution were obtained:The undoped parent phase BaFe O₃ presents helical antiferromagnetic（AFM）order below T_N≈117 K,and a small amount of Mn（5%）doping can destroy the helical magnetic（HM）structure,resulting in a ferromagnetic（FM）phase transition at T_C≈118 K.In the range of x=0.1～0.6,the long-range magnetic order cannot be observed due to competition between the AFM and FM interactions caused byMnincorporation.As doping contents ofMnincrease（x>0.7）,the solid solutions show AFM phase with increasing Néel temperature,because of the dominating Mn-O-Mn AFM interaction.Except for undoped BaFe O₃,which has semiconductor-to-metal transition,the doped components behave as semiconducting or insulating transport properties,and the insulation becomes stronger with the increasingMncontent.（2）A 4H-type BaMnO₃ single crystal was prepared by using two-step method at 5GPa and 1023 K.The crystal crystallizes to a hexagonal structure with space group P6₃/mmc.In this structure,face-sharingMnO₆ octahedral dimers connect with each other by corner O atoms along the c-axis direction,forming a-A-B-A-C-type 4H arrangement.A long-range AFM phase transition is found to occur at T_N≈263 K in 4H phase.In addition,when the synthesis pressure increases to 20 GPa,a new polymorphic phase was obtained.This higher-pressure phase still possesses the hexagonal P6₃/mmc symmetry,but a unit cell volume reduction by 2.05%.In this new phase,the c-axisMnO₆ dimers are separated byMnO₆ octahedral layers in ab plane,forming a-A-B-C-A-C-B-type 6H structure.The 6H phase exhibits two long-range AFM orderings at T_N1≈220 K and T_N2≈25 K,respectively.The magnetic properties of 4H and 6H phases are closely related to the crystal structures of the two phases.（3）Large-volume high-pressure single crystal BaFe_0.95Co_0.05O₃ was grown for the first time by a two-step method,and BaFe_0.95Mn_0.05O₃ single crystal doped with Mn-5%was compared for study.The results show that these two materials have the simple cubic Pm-3m perovskite structure,and Fe,Mn and Co are all+4 valence.We found that 5%doping of Mn⁴⁺and Co⁴⁺can change the HM structure of BaFe O₃,resulting in a long-range FM order transition at T_C≈118 K.However,Mn doping reduced the saturation magnetic moment of the material,because the total magnetic moment ofMnwas antiparallel to that of Fe.In addition,Co⁴⁺ions with the e_gelectron can lead to double exchange FM interactions within Fe⁴⁺ion,thus improving the saturation magnetic moment of the material.The introduction of Mn⁴⁺and Co⁴⁺can change the electrical transport properties of the parent phase.In contrast,Mn⁴⁺can significantly inhibit the itinerant electron behavior of the material system.